1. Field of the Invention
This invention relates to a semiconductor device and its manufacturing method especially suitable for use to various kinds of semiconductor devices using nitride III-V compound semiconductors.
2. Description of the Related Art
GaN semiconductors are direct-transitional semiconductors. Their bandgaps range from 1.9 eV to 6.2 eV, and they enable realization of light emitting devices capable of emitting light over visible regions to ultraviolet regions. For these reasons, they are attracting attention and are placed under vigorous developments. Further, these GaN semiconductors have a great possibility as materials of electron transport devices. That is, the saturation electron velocity of GaN is as large as approximately 2.5.times.10.sup.7 cm/s as compared with Si, GaAs and SiC, and its breakdown electric field is as large as approximately 5.times.10.sup.6 V/cm next to diamond. For these reasons, GaN semiconductors have been expected to have a large possibility as materials of electron transport devices for high frequencies, high temperatures and high powers.
As widely known, semiconductor devices are generally required to have a high resistance in regions other than device regions. For example, in a semiconductor laser having a stripe-shaped current path to concentrate a current and induce laser oscillation, currently used for making the current blocking structure are a method of growing semiconductor layers forming the laser structure, then making an insulation film on the surface thereof, and making a stripe-shaped window in the insulation film to use it as the current path, or a method of increasing the resistance of semiconductor layers other than the stripe portion by ion implantation. On the other hand, in electron moving devices, currently used are a method of fully removing conductive layers other than the device region by mesa etching or a method of locally increasing the resistance of the conductive layers by ion implantation. However, for semiconductor devices using GaN semiconductors, no optimum method has been established for increasing the resistance of regions other than device regions. Therefore, to date, devices using GaN semiconductors cannot perform their true characteristics.
Among the above-mentioned methods, the method of locally increasing the resistance of conductive layers by ion implantation is advantageous for making an IC because the high resistance region can be made in substantially the same plane as the device region. Actually, in most cases, isolation of devices in GaAs-based IC devices relies on this method for making a high resistance region by ion implantation. In Si-based devices, however, since the bandgap of Si is as small as 1.1 eV and acceptable insulation cannot be made by ion implantation, isolation of devices relies on pn junction.
As to semiconductor devices using GaN semiconductors, light emitting diodes have been brought into practical use, but semiconductor lasers or electron transport devices have not been realized yet for practical use. For semiconductor lasers and electron transport devices currently under developments, mesa etching is used for the former, and ion implantation for making a high resistance region or mesa etching is used for the latter. Among them, as to ion implantation for making a high resistance region, proposals given heretofore are briefly explained below.
Probably, the first report on ion implantation to GaN was Appl. Phys. Lett., 42, 430(1983), which used beryllium (Be) or nitrogen (N) as ion species. The true object of ion implantation in this report was to decrease the carrier concentration and increase the Schottky barrier height and not to isolate devices. Next reported in Appl. Phys. Lett., 63, 1143(1993) was an example using fluorine (F) as ion species of ion implantation for isolation of devices. Thereafter, also reported were examples using N and O as ion species for the same purpose (Appl. Phys. Lett., 66, 3042(1995) and J. Electron. Mater., 25, 839(1996)). These reports indicated that a difference appeared in resistance value among ion species O, N and F as a result of annealing and that a chemical difference appeared among defective species. Further reported were hydrogen (H) and helium (He) as ion species of ion implantation for isolation of devices (IEEE IEDM proceedings 96, 27(1996)).
As reviewed above, there have been proposed H, N, O, F and Be as ion species of ion implantation for making a high resistance region or for isolation of devices in semiconductor devices using GaN semiconductors. Among these ion species, complex defect of O is believed to make deepest levels and to be therefore optimum. From a chemical viewpoint, O is considered best, but there is no data on devises using it, and it is not yet a reliable technology. Although only H is used currently, H is liable to move when annealed, then diffuses into the device region, combines with donors or acceptors and inactivate them, decreases the carrier concentration, and invites deterioration of the device, for example. Therefore, H cannot be a good ion species. It was also reported that H as the ion species for making a high resistance region defects were annealed out due to a high temperature and recovered a conductivity (J. Appl. Phys., 78(5), 3008(1995)). Also, N, F and Be are not good ion species.